The development of diamond coatings on substrate materials applied by means of chemical vapor deposition (CVD) processes including hydrogen dissociation has led to different coating technologies and the provision of corresponding apparatus. Among the various techniques, preferred representatives are coating by the so-called hot filament (HF-CVD) process, by the microwave process and by the jet process including a plasma jet.
The various customary processes and their basic methods of operation have been summarized in a review article by B. Lux and R. Haubner in the Proceedings of the 12th International PLANSEE Seminar 1989, Volume 3, p. 615 ff, edited by H. Bildstein and H. Ortner. The review refers to 150 specialist papers, and thus a more detailed description of the customary techniques can be omitted here.
Among the various possible applications, the application of wear-reducing diamond layers to cemented cutting or machining inserts promises high technological utility and has already reached a high state of development. However, a significant problem which has only been insufficiently solved to date is the poor adhesion of the diamond layer to the substrate surface.
It has already been recognized that in a cemented material comprising a hard component and a binder metal, the binder metal found in a boundary zone close to the surface of the substrate is responsible for the poor adhesion properties. It is therefore not surprising that many processes have already been proposed for reducing the interfering influence of the binder metal at the surface.
EP 0 384 011 A1 gives, for the prior art, a review of measures which have hitherto been proposed for increasing the adhesive strength between a substrate material, in particular a cemented material, and a diamond layer deposited thereon. A first technique relates to the application of an intermediate layer, containing none of the binder metals iron, cobalt and/or nickel, between the cemented material and the diamond layer. However, the intermediate layer which can, from a practical point of view, only be applied to a limited thickness, does not provide a sufficient barrier to the migration of the binder to the coating surface. If, as proposed, a tungsten and carbon intermediate layer, for example, a W.sub.2 C-containing intermediate layer, is introduced, this brings with it the difficulty that different apparatuses for the deposition of the intermediate layer and the deposition of the diamond layer are additionally required.
According to another process variant, the application of a tungsten intermediate layer to a cemented cutting insert for metal cutting does substantially improve the adhesion problem. Since the W is converted at least partially into WC during the subsequent diamond coating, the intermediate layer is not a sufficient diffusion barrier.
According to a further previously described process, the cobalt in a cemented phase containing cobalt as binder metal is etched away from the surface of the cemented material by means of acid treatment. However, the etching can not be restricted to a boundary zone close to the surface, so that deep-reaching pores are formed there. A deep Co-depleted boundary zone leads to unacceptable brittleness in the boundary zone. The roughness and structure of the surface are unfavorable for subsequent coating. This greatly reduces the adhesion of the subsequent diamond layer.
According to a last process mentioned there, the surface of the cemented material is ground prior to the coating procedure. Nevertheless, surface regions of binder metal remain, on which a graphite phase instead of a diamond phase is still formed in the subsequent coating process, and graphite formation is a main cause of poor adhesion between substrate and diamond layer.
The process protected in EP 0 384 011 A1 directly follows this prior art. The grinding process is there followed by decarburization of the tungsten carbide/cobalt substrate surface with recrystallization of tungsten carbide. The decarburization is carried out by supplying oxygen gas to the substrate surface and proceeds simultaneously with the removal of metal, including in particular binder metal, from the surface, preferably as volatile oxide. A disadvantage of this process is the danger that the oxygen introduced leads to permanent, undesired oxide formation in some regions of the substrate. Grinding of the substrate surface necessarily causes the deposition there of impurities which cannot be completely removed during the subsequent decarburization process without simultaneous, lasting impairment of the surface structure or the surface roughness with regard to good adhesion of diamond layers.
A significant disadvantage of this process is finally the requirement for suitable equipment besides the equipment for the CVD diamond coating. Thus, the Ta or W filaments customarily used in hot filament equipment would become unusable as a result of oxygen treatment.
It is an object of the present invention to lastingly improve the adhesion improvement between a cemented carbide substrate and a diamond layer applied thereto, without incurring the above mentioned disadvantages in the use of the known processes. The quality of adhesion to be achieved should be viewed, in particular, in the context of the use of products coated in this way as a cutting insert or as a machining tool, where, because of high local thermal and mechanical stresses, particularly high demands are made in this respect.